The present invention relates to novel hydrophilic copolymers and soft contact lenses made therefrom.
A hydrophilic polymer suitable for use as an extended wear soft contact lens must have high oxygen permeability and good mechanical strength.
The characteristics of an extended wear contact lens include the following:
1. Tolerance of the eye to a foreign body such as a contact lens: PA0 2. Maintenance of a physiological norm with such a lens in place: PA0 3. Good optical properties:
Tolerance of a soft lens by the ocular environment is affected by water content, shape, or geometry, thickness and edge quality. The surface of the lens should be smooth in order not to affect the normal precorneal tear film and the superficial epithelial layer. Such a smooth surface should not be affected by temperature, pH, tonicity and foreign body deposition, mucous adhesion, lipids of the tears or abrasion by the lids. Wettability of the lens surface is important in order to allow tears to fill the lens-corneal interspace. The lens should also be physiologically inert.
Proper oxygenation is essential for the aerobic cycle of metabolism of the cornea. It has been clinically established that the minimum corneal requirement of oxygen is 3.5 .mu.l (STP)/cm.sup.2 hr. Oxygen permeability of the lens will depend on the type of material (chemical composition), water content and thickness. Basically, it is agreed that the oxygen prerequisite of the cornea could be met by a lens with about 60% water content and less than 0.2 mm thick.
Some constant wear patients go through a phase of morning corneal edema which clears after waking. These symptoms are slight and in some cases disappear after a few weeks of wearing.
In view of the corneal metabolism, the chemical and physical properties of the lens are very important. The hydrophilic lens should not selectively absorb by-products of metabolism, toxins and enzymes, concentration of which might eventually lead to tissue damage.
The material of the lens should be optically clear with refractive index close to 1.43 (refractive index of tears), and should be chemically and physically stable in order to retain its optical qualities.
Practically all, if not all, commercially available soft contact lenses that could be considered for extended wear incorporate HEMA (2-hydroxyethyl methacrylate) in compositions with other monomers of high hydrophilicity, methacrylic acid and N-vinylpyrrolidone, or are made of silicone rubber.
The group of lenses using highly hydrophilic monomers face several important problems. Mechanical strength of these high water content copolymers is inferior and the lens in order to withstand handling has to be made thick. Since oxygen permeability is directly related to membrane thickness, the advantage of high water content is cancelled out by the thickness.
Deposits on lenses are also a great drawback to continuous wear. Therefore, methacrylic acid, which has a negatively charged carboxyl group on the molecule, will attract oppositely charged particles contributing to lens contamination.
Another widely used material to improve hydrophilicity is N-vinylpyrrolidone. Tensile strength of these copolymers is again very low and moreover N-vinylpyrrolidone decreases heat stability of the system. Lenses with high N-vinylpyrrolidone content cannot be sterilized by autoclaving without permanent damage and discolorization.
Silicon has a superior oxygen permeability to the hydrogels due to the flexibility of the Si-O bond. Silicon rubber, however, is strongly hydrophobic and therefore quite irritating for the lens wearer. The second major problem is the ability of silicones to absorb lipophilic substances.
An object of the present invention is to provide a hydrogel lens which would combine the following properties:
Water content: approximately 60% PA1 Burst Strength/0.1 mm (wet): comparable to commercial Hydron.RTM. lenses (made from polyHEMA PA1 Total Extractables: 10% maximum PA1 Oxygen Permeability: at least twice better than polyHEMA PA1 Good optical properties PA1 Ability to withstand autoclaving PA1 Ability to withstand routine cleaning and disinfection regimen PA1 1. Low temperature curing for IPP compositions: 22 hours at 40.degree. C. followed with two hours postcuring at 80.degree. C., PA1 2. High temperature curing for Percadox 16 initiated compositions: 2 hours are 95.degree. C.